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pre-lecture-slides.pdf - 2022-03-15 Hydrogen fusion in...

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2022-03-151Hydrogen fusion in stars with massmore than 2 solar massThe proton-proton chain that isresponsible for hydrogen fusion inthe Sun still occurs.In stars with a denser, hotter corethan the Sun, Carbon, Nitrogen andOxygen nuclei can fuse with protonsin a complicated cycle that has thesame result as the proton-protonchain:4 hydrogen become 1 heliumThis process is called theCNO cycle.It allows massive stars to increasethe rate of fusion in their cores.High mass stars running out of hydrogenHigh mass stars stop burning hydrogen in their core in a few million years.Intermediate mass stars just take a longer time to run out of hydrogen in their cores.Helium builds up in the core and gravity squeezes it smaller. Hydrogen fusioncontinues in a shell around the core. The outer layers expand (same as low massstars) and the star becomes a supergiant.The contraction of the core has raised the temperature high enough to graduallystarthelium fusion.There is no helium flash in massive stars.For high mass stars, helium fusion is shorta few hundred thousand years only.Helium fusion creates carbon through the triple-alpha process. The carbon builds upto form a carbon core with no nuclear fusion, surrounded by 2 shells.Gravitysqueezes the carbon core smaller.12
2022-03-152High mass stars after helium fusionIn high mass stars (mass more than 8 solar masses) the contractingcarbon core gets hot enough to reignite fusion before it becomesdegenerate.Carbon fusion requires a temperature of 600 million K, that’s 40 timeshotter than the centre of the Sun.When the conditions for carbon fusion is reached, many differentfusion reactions can occur.Some produce helium as a by product, thatcan be used in other fusion reactions involving carbon and oxygen.Here are some nuclear fusionreactions that happen in thecores of massive stars.Carbon and oxygen can capturehelium nuclei to formprogressively more massivenuclei (neon, magnesium)Carbon-carbon, carbon oxygenand oxygen oxygen fusionmakes elements like silicon andsulfur.In the most massive stars, thiswill only last for centuries.When the core gets hot enough,silicon fusion forms iron.The fate of the star is nowsealed.34
2022-03-153Last stage of evolution of massive starsBuild-up of an inert iron core.Silicon fusion into iron may only take a few days.Why iron in the core means the end of the road:Recall that energy is releasedwithfusion of small nuclei intolarger nucleifission of very large nucleiinto smaller nucleiSomewhere in the middle, theremust be a situation in whichneither fusion, nor fissionreleases energy.The element in the middle is iron56
2022-03-154Iron fusion costs energyA long time ago there wasonly H in the universe -where do heavier elementslike lead, gold and uraniumcome from?

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Term
Spring
Professor
KIM
Tags
Black hole, Supernova, White dwarf, Neutron star

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